Parallel steam reformers to provide low energy process

ABSTRACT

A low energy process for the production of hydrogen-rich gas which involves the process sequence of primary reforming and secondary reforming includes parallel steam reformers for the primary reforming of the hydrocarbon feed, one portion of the hydrocarbon feed being heated using radiant heat, i.e., a steam reforming furnace, and another portion of the hydrocarbon feed being heated using indirect heat exchange with the effluent from the secondary reforming, i.e., an exchanger-reactor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to an improved process and system forthe production of hydrogen-rich gas by the steam reforming ofhydrocarbons. More specifically, the present invention involves a lowenergy process and system for the production of ammonia synthesis gasincluding the process steps of primary and secondary reforming forproducing hydrogen-rich gas which includes the parallel primaryreforming of the hydrocarbon feed, one portion of the hydrocarbon feedbeing heated using radiant heating and another portion being heatedusing indirect heat exchange with the effluent from the secondaryreforming.

2. Prior Art

U.S. Pat. No. 3,094,391 discloses a steam reforming furnace having areaction radiant-heating section and a reaction convection-heatingsection. A portion of the hydrocarbons is passed to tubes in theradiant-heating section of a furnace, and the balance of thehydrocarbons is passed in parallel to tubes provided in the reactionconvection-heating section for steam reforming.

A conventional steam reforming furnace is disclosed in U.S. Pat. No.3,257,172 and in a conventional process, as disclosed in U.S. Pat. No.3,441,393, the steam reforming furnace is the sole system for carryingout the steam reforming reaction.

U.S. Pat. No. 3,549,335 discloses a reactor wherein the hydrocarbons aresteam reformed in the lower portion of an inner shell having a containerfor secondary catalyst thereabove and a process for primary andsecondary reforming in the same reactor is carried out.

U.S. Pat. No. 3,751,228 discloses an apparatus for reforminghydrocarbons under pressure comprising a heat-exchange chamberincorporating reaction tubes for effecting a process of primaryreforming and a shaft chamber for effecting a process of secondaryreforming located at the open ends of the reaction tubes.

U.S. Pat. No. 3,958,951 discloses a reformer furnace having a convectionsection, means for preventing radiant heat from the burners to theconvection section, a centrally disposed effluent tube suspended fromthe top of the furnace, a tube sheet surrounding and suspended from thecentrally disposed effluent tube and reformer tubes suspended from thetube sheet which reformer tubes are in communication with the interiorof the effluent tube.

U.S. Pat. No. 3,870,476 discloses a pressure vessel for catalyticendothermic reactions in the upper portion of which is a supportingplate for a plurality of open ended shell tubes within which arereaction tubes.

SUMMARY OF THE INVENTION

In a process and system for the production of a hydrogen-rich gas fromhydrocarbons which comprises the sequence of primary reforming followedby secondary reforming, the improvement which comprises in parallelheating to primary reforming conditions a first mixture of thehydrocarbons and steam by radiant heating and heating to primaryreforming conditions a second mixture of the hydrocarbons and steam byindirect heat exchange with the effluent from the secondary reformingprocess and reforming in the presence of a steam reforming catalyst,then combining the primary reformed effluents and introducing thecombined effluents to the secondary reforming process to form ahydrogen-rich gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram which illustrates one embodiment of the presentinvention; and

FIG. 2 is a specific embodiment of the present invention which includesa steam reforming furnace with both a radiant heat section and aconvective heat section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to an improved process for reforminghydrocarbons by the sequence of primary reforming followed by secondaryreforming. More specifically, the present invention involves separatingthe hydrocarbon feed into parallel streams and, after preheating,heating a portion of the hydrocarbon feed to primary reformingconditions by radiant heating and heating another portion of thehydrocarbon feed to primary reforming conditions by indirect heatexchange with the effluent from the secondary reforming process. Thepresent invention enables the design of a low energy synthesis processsince a considerable amount of the heat necessary for the primary steamreforming is obtained from the process, and therefore, the overall fuelconsumption can be reduced to obtain the desired primary reforming.Heretofore, the primary reforming process has been a large fuelconsumption process where the heat introduced to the process stream by aconventional radiant steam reforming furnace has been recovered afterthe secondary reforming process in the form of steam by heating waterwhich is at temperatures well below the temperatures used for primaryreforming. The present invention has specific applicability to a lowenergy ammonia process where the requirement for the large amount ofsteam usually generated in a conventional process has been materiallyreduced, and thus an overall low energy ammonia process is achieved byreducing the fuel consumption in the primary reforming process whilestill generating the desired amount of steam.

The production of a hydrogen-rich gas or synthesis gas by steamreforming is known. The initial reaction which takes place isillustrated by the equation:

    C.sub.x H.sub.y + H.sub.2 O → xCO + (x + y/2)H.sub.2

    ch.sub.4 + h.sub.2 o → co + 3h.sub.2

this endothermic reaction is reversible and gives way to the exothermicmethane forming reaction illustrated by the following:

    CO + 3H.sub.2 → CH.sub.4 + H.sub.2 O

thus, the product gas of the primary steam reforming process is referredto as a partially reformed or hydrogen-rich gas since the completeconversion of hydrocarbons to hydrogen and carbon monoxide is notaccomplished.

In the primary steam reforming process, hydrocarbons are contacted withsteam in the presence of a steam reforming catalyst under conditionswherein the rate of production of a hydrogen-rich gas is favored.Gaseous hydrocarbons, or liquid hydrocarbons which can be gasified, arethe feed material, such as natural gas, ethane, propane, LPG or naphthasboiling in the gasoline range and the like. The steam reformingcatalysts which may be employed are nickel, nickel oxide, chromia,molybdenum, mixtures thereof, etc. The details of steam reformingcatalysts are known, as well as the operating conditions, see forexample U.S. Pat. No. 3,119,667, the specifics of which do not form abasis of the present invention. The steam reforming reaction is carriedout at steam to carbon ratios of 2:1 to 6:1 and preferably 3:1 to 4.5:1such that the outlet temperatures are maintained at between about 730°C. and 925° C., and preferably between about 790° C, and 870° C. Thepressure in the steam reforming process may range from about atmosphericpressure to 75 atmospheres (1100 psi) and preferably between 20 and 60atmospheres. In a specific embodiment of the present invention, thepressure is preferably between about 35 and 45 atmospheres.

The heating of the hydrocarbon feed to conditions at which the primaryreforming occurs is usually carried out in a tube. Thus, specifictemperatures to which the hydrocarbon feed is heated are governed bytube size and the metallurgy of the tube. In a specific embodiment ofthe invention, the tubes are filled with the steam reforming catalyst,conventionally a nickel oxide on an inert support which is reduced insitu with hydrogen, whereby the heating is through the tube wall andreaction occurs within the tube. However, according to the presentinvention, the heating of the hydrocarbon feed to conditions at whichprimary reforming occurs may be carried out prior to being in thepresence of the steam reforming catalyst, usually in a multiplicity ofheating and catalyst contacting steps, commonly referred to as adiabaticsteam reforming (see U.S. Pat. No. 3,771,261), and such adiabaticreforming may occur in one parallel stream and not in another.

Unlike the prior art where a single piece of equipment, such as a steamreforming furnace or a special reformer, is suggested to carry out theprimary steam reforming, the present invention uses a conventionalreformer furnace and an exhanger-reactor such that the hydrocarbon feedcan be divided and partially reformed as parallel streams. According tothe present invention, about 15 to 60% volume of the hydrocarbon feed ispassed to the exchanger-reactor while 85 to 40% by volume is fed to theconventional reformer furnace. In a preferred range, about 25 to 50% byvolume of the hydrocarbon feed is fed to the exchanger-reactor. In aspecific embodiment of the invention, the reformer furnace will containnot only a radiant heating section but also a convective heating sectionsuch that the heating of three parallel feeds can be accomplished. Thethird stream passed through the convective heating section may containbetween 5 and 20% by volume of the hydrocarbon feed. The partiallyreformed gas obtained from each of the parallel streams may be combinedfor secondary reforming. The conditions in each of the parallel streamsmay be different, such as having different amount of steam or differentoutlet temperatures, and accordingly, some difference in reforming ineach of the streams may occur. The composition of the partially reformedgas from each of the parallel streams may thus be different.

Following primary reforming, the combined partially reformed gas isfurther reacted to form additional hydrogen by secondary reforming. Thepartially reformed gas is reacted with oxygen, usually as air, andadditional steam, if required, in the presence of a catalyst maintainedat elevated temperatures and at approximately the pressure of theprimary reforming process. The conditions in the secondary reformer aresuch that the temperatures are maintained at an outlet temperaturebetween about 870° C. and 1075° C, preferably from about 910° to 1020°C. Air is preferably employed especially for ammonia production, toprovide the oxygen requirement of the secondary reforming because of itslow cost and availability, but it should be understood that oxygen oroxygen-enriched air can be used. Suitable secondary reforming catalystsare nickel, nickel oxide, cobalt oxide, chromia, molybdenum oxide, etc.The preferred catalyst is nickel. The conventional sequence of primaryreformer followed by secondary reformer, and the details of thesecondary reformer are described in more detail in U.S. Pat. No.3,441,393.

The effluent from the seconday reformer is a stream containing largequantities of heat and according to the present invention is utilized toprovide the heat to an exchanger-reactor. Since a substantial portion ofthe hydrocarbon feed can be passed through such an exhanger-reactor, thefuel normally consumed in a steam reforming process can be reducedsubstantially. A reduction of 25 to 100 NMBTU/HR of radiant reformingduty to reform 2600 mols/hr. of a natural gas hydrocarbon feed is madepossible by using the parallel reformer system of the present invention.

The hydrogen and hydrogen-rich gases may be used in many importantprocesses, e.g., mixtures of hydrogen and carbon monoxide are employedin the synthesis of hydrocarbons and of oxygenated hydrocarbons, such asalcohols or ketones. Many known petroleum refining processes such ashydrodesulphurization require hydrogen. A most important usage for thehydrogen-rich gas produced according to the present invention is inammonia synthesis.

For a better understanding of the present invention, reference is madeto the following examples and specific embodiments as shown in thedrawings.

It will be understood that various valves, pumps, controls and relatedauxiliary equipment may be necessary in practicing the presentinvention. In the interest of simplicity, such items have not been shownor described since the need for them, their location and their manner ofuse are well known to those skilled in the art.

Referring to FIG. 1, a specific embodiment is illustrated which employsa conventional steam reforming furnace which is in parallel with anexchanger-reactor, wherein additional primary steam reforming is carriedout according to the present invention. A hydrocarbon feed, which may benatural gas, ethane, propane, or naphtha is introduced in line 11 andpreheated to about 370° to 430° C. preferably about 400° C. in preheater12, which may be in the convection section of a primary reformer furnace13. The gaseous hydrocarbon feed is then passed by line 14 to apre-treater 15. The hydrocarbon feed may require pre-treatment toeliminate or decrease the concentration of undesirable components whichmay have a deleterious effect on subsequent processing steps. Forexample, many hydrocarbon feeds contain sulphur which is a steamreforming catalyst poison. In such a case, pre-treater 15 is a knowndesulphurizer such as a zinc oxide guard chamber. The effluent flowsfrom the pre-treater 15 by line 16 and is mixed with steam introduced byline 17. The combined hydrocarbon feed and steam, is at about 350° C.and a pressure of 1 to 75 atmospheres.

In accordance with this embodiment of the present invention, thecombined hydrocarbon feed and steam stream is divided into parallelstreams and introduced in lines 18 and 19, respectively. Alternatively,the hydrocarbon feed may be divided into parallel streams with steamadded to each stream. The steam to carbon ratio in each parallel streammay then be different. A portion of the hydrocarbon feed (40 to 85% byvolume) and steam is introduced into line 18 wherein the mixture ofhydrocarbon feed and steam is preheated in heat exchanger 20, which maybe an exchanger within the convection section of the reformer furnace13, and then the mixture is introduced by line 21 to a plurality ofsteam reforming tubes 22 in rows in the primary reformer furnace 13heated by means for producing radiant heat such as down-fired burnersbetween the rows of tubes 22 or side-fired burners. The steam reformingtubes 22 are filled with convectional steam reforming catalyst such as acommercial nickel catalyst. The effluent, a partially reformed gas,flows from tubes 22 by line 23 for introduction to the secondaryreformer 24. Process air and steam are introduced by line 25 tosecondary reformer 24 to carry out the secondary reforming. Anothermixture of the hydrocarbon feed (15 to 60% by volume) and steam isintroduced into line 19 wherein the mixture of hydrocarbon feed andsteam is preheated in heat exchanger 26, and then introduced by line 27to an exchanger-reactor 28. The exchanger-reactor 28 contains tubes 29filled with steam reforming catalyst in one specific embodiment. Themixture of hydrocarbon feed and steam is passed through the plurality oftubes 29 and the effluent, a partially reformed gas, flows from thetubes 29 of exchanger-reactor 28 by line 30. The partially reformed gasin line 30, which may differ in composition, is combined with thepartially reformed gas in line 23 from the primary reformer furnace andintroduced into the secondary reformer 24.

According to the present invention, the effluent from the secondaryreformer 24 is used to supply the heat of reaction for the mixture ofhydrocarbon feed and steam by indirect heat exchange as the mixture ispassed through the exchanger-reactor 28 to carry out primary reformingof the hydrocarbons in the mixture. In a specific embodiment, theexchanger-reactor 28 may be a tube and shell heat exchanger. Theeffluent or reformed gas from the secondary reformer 24 is passed byline 31 and introduced on the shell side of exchanger-reactor 28 to heatthe hydrocarbons in the tubes 29. The reformer gas exits theexchanger-reactor 28 by line 32 whereby the hydrogen-rich gas may beused as such or further processed in a known manner. The tubes 29 may bepartially filled or filled to substantially their entire length withstream reforming catalyst. On the other hand, the catalyst and streambeing reformed may be on the shell side while the reformed gas may bepassed through the tubes. Also the exchanger-reactor 28 may comprise amultiplicity of heating and catalyst contacting stages for adiabaticreforming. The pressure difference between the mixture of hydrocarbonfeed and steam in tubes 29 and the reformed/gas on the shell side ofexchanger-reactor 28 is small, thus allowing thin wall tubes to be usedin the exchanger-reactor 28. However, the shell side of theexchanger-reactor 28 must be designed for the pressure at which thereforming process is being carried out. At the preferred processpressures, the exchanger-reactor 28 is a high pressure vessel.

Referring to FIG. 2, a specific embodiment is illustrated which employsa steam reforming furnace wherein the steam reforming tubes are locatedin the convection section as well as the radiant section of the steamreforming furnace. A hydrocarbon feed is introduced in line 51 andpreheated in preheater 52, which may be a heat exchanger in theconvection section of the furnace 53. The gas is then passed by line 54to a pre-treater 55. The effluent from the pre-treater 55 is removed byline 56 and is mixed with steam which is introduced by line 57.

In this specific embodiment the mixed stream of hydrocarbons and steamis introduced in parallel to lines 58, 59 and 60, respectively. Theportion of hydrocarbon feed (35 to 80% by volume) and steam introducedin line 58 is preheated in heat exchanger 61 which may be in theconvective heating section of the furnace 53, and then the mixture isintroduced by line 62 into a plurality of tubes 63 in rows in theradiant section 64 of the furnace 53 having burners (not shown) forproducing radiant heat. The tubes 63 are filled with commercial steamreforming catalyst. On the other hand, an adiabatic reformer may be usedto heat the mixture with radiant heat. The effluent from the tubes 63 orpartially reformed gas is passed by line 65 to the secondary reformer66. In this embodiment a portion of the hydrocarbon feed (5 to 20% byvolume) and steam is passed by line 59 to heat exchanger 67 where themixture is preheated and then by line 68 to a plurality of tubes 69which are in the convection section 70 of the steam reforming furnace53. The mixture of hydrocarbon feed and steam in tubes 69 are at thesteam reforming process pressure whereas the pressure outside tubes 69is essentially ambient pressure. The tubes 69 are also filled with steamreforming catalyst. The effluent from the tubes 69 is passed by line 71where it is combined with the other partially reformed gases andintroduced to the secondary reformer 66. A third mixture of thehydrocarbon feed (15 to 60% by volume) and steam is passed by line 60into heat exchanger 72 in the convection section of the furnace 53 wherethe mixture is preheated and then by line 73 into an exchanger-reactor74. This mixture of hydrocarbon feed and steam is passed through thetubes 75 which may contain steam reforming catalyst wherein thehydrocarbons are partially reformed to hydrogen and carbon monoxide. Theeffluent from the tubes 75 is passed by line 76 where it is combinedwith the other partially reformed gases and introduced into thesecondary reformer 66.

Also introduced into secondary reformer 66 by line 77 is a mixture ofair and steam to carry out the secondary reforming reaction. Theeffluent from the secondary reformer 66, the reformed gas, is passed byline 78 into the shell side of exchanger-reactor 74 to provide thenecessary heat for the reforming of the hydrocarbons and steam in thetubes 75. The reformed gas exits the exchanger-reactor 74 by line 79.

As a specific example, a hydrocarbon feed of natural gas, 2636.9mol/hr(MPH), is introduced in line 51 and preheated to about 400° C. inpreheater 52. The gas is then passed by line 54 through a zinc oxideguard chamber 55. After treatment, the gas is removed by line 56 whereit is mixed with 10,004 MPH of steam introduced by line 57. The mixedstream of hydrocarbons and steam is divided into three separate portionsfor primary reforming. One portion of the mixed hydrocarbons and steam,5942.5 MPH or about 47%, is introduced in line 58 where it is heated inheat exchanger 61 to about 500° C. and then passed through a pluralityof tubes 63 in the radiant section of steam reforming furnace 53. Theconditions of the partially reformed gas from the tubes 63 at the outletof the furnace 53 is about 850° C. and a pressure of about 47atmospheres. Another portion of the mixed hydrocarbons and steam, 1569.9MPH or about 12%, is introduced in line 59 to heat exchanger 67 forpreheating the mixture and then by line 68 to tubes 69 which are in theconvection section 70 of the steam reforming furnace 53. The effluent orpartially reformed gas from tubes 69, having an outlet temperature ofabout 850° C. and a pressure of about 47 atmospheres, is combined withthe gas from tubes 63. A third portion of the mixed hydrocarbons andsteam, 5128.5 MPH or about 41% is introduced in line 60 and passed toheat exchanger 72 for preheating the mixture. This mixture is thenpassed to exchanger reactor 74 where it is passed through tubes 75 whichalso contain a commerical nickel steam reforming catalyst. The effluentfrom tubes 75 has an outlet temperature of about 811° C. and a pressureof about 47 atmospheres. The combined effluents or partially reformedgas, 16,127.5 MPH is introduced to the secondary reformer 66 at atemperature of about 808° C. Also introduced to the secondary reformer66, is a combined stream of air, 4122.9 MPH (wet), and steam 389.7 MPH,at a temperature of about 693° C. The effluent from the secondaryreformer 66 has a temperature of about 979° C. which is passed by line78 into the shell side of exchanger-reactor 74 to provide the necessaryheat for the reforming of the hydrocarbons in tubes 75.

The parallel reforming of the present invention allows the adjustment inthe steam production over the present day commercial primary reformingprocess. In the production of ammonia, an estimated savings of 2 to 6MMBTU/ST of ammonia is possible by using the parallel reforming of thepresent invention. In other processes, the savings may be fromtwenty-five to over fifty percent of the primary reforming duty inBTU/HR.

The nature and objects of the present invention having been completelydescribed and illustrated and the best mode thereof set forth, what wewish to claim as new and useful and secure by Letters Patent is:
 1. In aprocess for the steam reforming of hydrocarbons which comprises thesequence of primary and secondary reforming, the improvement whichcomprises:in parallela. heating a first mixture of hydrocarbon feed andsteam to conditions at which primary reforming occurs by radiant heatingand reforming said hydrocarbons in the presence of a steam reformingcatalyst to form a first partially reformed effluent, b. heating asecond mixture of hydrocarbon feed and steam to conditions at whichprimary reforming occurs by indirect heat exchange with process gas ashereinafter defined and reforming said hydrocarbons in the presence of asteam reforming catalyst to form a second partially reformed effluent,introducing said first and second partially reformed effluents to asecondary reformer to carry out said secondary reforming in the presenceof oxygen and forming a secondary reforming effluent, and passing saidsecondary reforming effluent as the process gas in indirect heatexchange with said second mixture of hydrocarbon feed and steam as setforth in (b) above.
 2. A process according to claim 1 wherein saidhydrocarbons are heated and reformed in tubes containing said steamreforming catalyst.
 3. A process according to claim 1 wherein at leastone mixture of hydrocarbon feed and steam is first heated and then isreformed adiabatically in the presence of a steam reforming catalyst. 4.A process according to claim 1 wherein 15 to 60% by volume of saidhydrocarbon feed is heated by indirect heat exchange with the effluentfrom said secondary reforming.
 5. A process according to claim 1 wherein25 to 50% by volume of said hydrocarbon feed is heated by indirect heatexchange with the effluent from said secondary reforming.
 6. A processaccording to claim 1 wherein said reforming process pressure is about 20to 60 atmospheres.
 7. In a process for the steam reforming ofhydrocarbons which comprises the sequence of primary and secondaryreforming, the improvement which comprises:in parallela. heating a firstmixture of hydrocarbon feed and steam to conditions at which primaryreforming occurs by radiant heating and reforming said hydrocarbons inthe presence of a steam reforming catalyst to form a first partiallyreformed effluent, b. heating a second mixture of hydrocarbon feed andsteam to conditions at which primary reforming occurs by convectiveheating at essentially ambient pressure and reforming said hydrocarbonsin the presence of a steam reforming catalyst to form a second partiallyreformed effluent, c. heating a third mixture of hydrocarbon feed andsteam to conditions at which primary reforming occurs by indirect heatexchange with process gas as hereinafter defined and reforming saidhydrocarbons in the presence of a steam reforming catalyst to form athird partially reformed effluent, combining said first, second andthird partially reformed effluents and introducing said effluents to asecondary reformer to carry out said secondary reforming in the presenceof oxygen and forming a secondary reforming effluent, and passing saidsecondary reforming effluent as a process gas in indirect heat exchangewith said third mixture of hydrocarbon feed and steam as set forth in(c) above.
 8. A process according to claim 7 wherein said hydrocarbonsare heated and reformed in tubes containing said steam reformingcatalyst.
 9. A process according to claim 7 wherein at least one portionof said hydrocarbon feed is first heated and then is reformedadiabatically in the presence of a steam reforming catalyst.
 10. Aprocess according to claim 7 wherein 5 to 20% by volume of saidhydrocarbon feed is heated by convective heating at essentially ambientpressure.